Resonant microcavity display

ABSTRACT

A resonant microcavity display ( 20 ) having microcavity with a substrate ( 25 ), a phosphor active region ( 50 ) and front and rear reflectors ( 30  and  60 ). The front and rear reflectors may be spaced to create either a standing or treaveling eledtromagnetic wave to enhance the efificenty of the light transmission.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a luminescent screen comprisinga resonant microcavity having a phosphor active region.

[0003] 2. Description of the Prior Art

[0004] Conventional cathode ray tube (CRT) displays use electronsemitted from an electron gun and accelerate them through an intenseelectric field Projecting them onto a screen coated with a phosphormaterial in the form of a powder. The high-energy electrons exciteluminescence centers in the phosphors which emit visible light uniformlyin all directions. CRT's are well established in the prior art and arecommonly found in television picture tubes, computer monitors and manyother devices.

[0005] Displays using powder phosphors suffer from several significantlimitations, including: low directional luminosity (i.e., brightness inone direction) relative to the power consumed; poor heat transfer anddissipation characteristics; and a limited selection of phosphorchromaticities (i.e., the colors of the light emanating from the excitedphosphors).

[0006] The directional luminosity is an important feature of a displaybecause the directional properties influence the efficiency with whichit can be effectively coupled to other devices (e.g., lenses forprojection CRT's). The normal light flux pattern observed from aluminescent screen closely follows a “Lambertian distribution”; i.e.,light is emitted uniformly in all directions. For direct viewingpurposes this is desirable, as the picture can be seen from all viewingangles. However, for certain applications a Lambertian distribution ofthe light flux is inefficient. These applications include projectiondisplays and the transferring of images to detectors for subsequentimage processing.

[0007] Heat transfer and dissipation characteristics are importantbecause one of the limiting factors in obtaining bright CRT's suitablefor large screen projection is the heating of the phosphor screen. Asthe incident electron beam density increases, the phosphor temperatureincreases. When the phosphor reaches a certain temperature, itsluminosity decreases. This is known as thermal quenching. Withconventional powder-phosphor displays the phosphor-to-screen heattransfer characteristics are relatively poor, therefore heat dissipationis limited and thermal quenching can occur at relatively low electronbeam densities. Because projection displays require high electron beamdensities to produce the brightness required to project an image, thisinefficiency makes conventional CRT's poorly suited for projectiondisplays.

[0008] Chromaticity is important because the faithful reproduction ofcolors in a display requires that the three primary-color phosphors(red, green and blue) conform to industry chromaticity standards (e.g.,European Broadcasting Union specifications). Finding phosphors for eachof the three primary colors that exactly match these specifications isone of the most troublesome aspects of phosphor development.

[0009] The decay time of the activator (i.e., light emitting ion in thephosphor) is also another important parameter for a phosphor. In anideal phosphor for high brightness applications, it is desirable tocontrol directly the decay time of the phosphor for each displayapplication. For example, in some applications, shorter decay timesallow rapid re-excitation of the activator with a corresponding increasein the maximum light output. The decay time is typically determined bythe natural spontaneous transition rate of the activator. In order toimprove phosphor performance it is therefore desirable to have controlover this spontaneous transition rate.

[0010] Another problem encountered in conventional phosphor displays isthat energy can transfer from one activator to another nearby activatorin the phosphor host matrix. This is a nonradiative process where theefficiency of the phosphor is reduced. The energy transfer increaseswith increasing activator concentration and therefore it limits thedensity of activators that can be incorporated in a display and thus themaximum light output.

[0011] The use of a single-crystal, thin-film phosphor as a faceplatefor a CRT was first described in a British patent application by M. W.Van Tol, et al., UK Pat. GB-2000173A (1980). This patent taught the useof an yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) film grown by liquid phaseepitaxy (LPE) on a single-crystal YAG substrate. The YAG film is dopedwith a rare-earth ion which emits light when excited by electrons.(Doping is the process wherein dopant ions are substituted for host ionsin the crystal lattice during crystal growth.) In this device, thethickness of the thin-film layer is from one to six microns and does notbear any relation to the wavelength of the light to be emitted by thedisplay.

[0012] This device exhibited several advantages over conventionalpowder-phosphor displays. One such advantage was that heat wastransferred from the phosphor more efficiently because of the perfectcontact between the phosphor and the screen, and because of the highthermal conductivity of the YAG substrate. The screen could be loadedwith a higher beam density without exhibiting thermal quenching and,therefore, could produce more light.

[0013] Another advantage of single-crystal phosphor luminescent screensversus powder deposited luminescent screens is concerned with theresolution of a pixel (i.e., light producing spot). For high resolutiondisplays using powder phosphor, the limiting size of a pixel—and hencethe resolution of the screen—is determined by the particle size of thephosphor powder. Single-crystal phosphors, on the other hand, are notaffected by this since they do not contain discrete particles.

[0014] Powder phosphors further reduce resolution due to the lightscattering from the surface of the powder. Because of the lack ofdiscrete phosphor particles and the absence of light scattering,thin-film displays have high image resolution, limited only by the spotsize of the exciting electron beam. The increasing demand for higherresolution displays makes this a particularly attractive advantage.

[0015] Yet another advantage is concerned with producing a vacuum in aCRT. To allow the electron beam to travel between the electron gun andthe phosphor screen, a vacuum must be maintained within a CRT.Conventional powder phosphors have a high total surface area and,generally, organic compounds are used in their deposition. Both the highsurface area and the presence of residual organic compounds causeproblems in holding and maintaining a good vacuum in the CRT. Usingthin-film phosphors overcomes both of these effects, as the totalexternal surface area of the tube is controlled by the area of thethin-film (which is much less than the surface area of a powder phosphordisplay) and, furthermore, there are no residual organic compoundspresent in thin-film displays to reduce the vacuum in the sealed tube.

[0016] The thin-film phosphors of Van Tol, et al., exhibit oneprohibiting disadvantage, however, due to the phenomenon of “lightpiping.” Light piping is the trapping of light within the thin-film,rendering it incapable of being emitted from the device. This is causedby the total internal reflection of the light rays generated within thethin-film. Since the index of refraction (n) of most phosphors is aroundn=2, only those light rays whose incident angles are less than thecritical angle, θ_(c) (where sin θ_(c)=1/n) will be emitted from thefront of the thin-film. The critical angle for an n=2 material is around30°. Therefore, the fraction of light that escapes from the front of thethin-film is only about 6.7% of the total light. The common design ofplacing a highly reflective aluminum layer behind the film only doublesthe output to about 13% of the light. Moreover, this light is spread ina “Lambertian distribution” and is not directional. As a result of lightpiping, the external efficiency (i.e., the percentage of photonsescaping from the display relative to all photons created in thedisplay) is less than one-tenth that of powder phosphor displays.Therefore, in spite of the unique advantages offered in terms of thermalproperties, resolution, and vacuum maintenance; the development ofcommercial CRT devices based on thin-films is held back by their poorefficiency due to “light piping”.

[0017] Some schemes have been designed to reduce the “light piping”problem. One scheme described by Bongers, et al., U.S. Pat. No.4,298,820 (1981), uses a thin-film, deposited by LPE, with V-shapedgrooves etched into the surface to reflect light out of the thin-film.This approach brought about an improvement in external efficiency ofaround 1½ to 2½ times that of a thin-film display without the V-shapedgrooves. Given the previous external efficiency of 13%, this would stillonly lead to a total external efficiency of around 20% to 30%.

[0018] Another scheme, described by Huo and Hou, “ReticulatedSingle-Crystal Luminescent Screen”, 133 J. Electrochem. Soc. 1492(1986), involves etching individual mesa shapes onto the thin-filmdeposited by LPE. This led to a three times improvement in externalefficiency (still rendering only about a 30% external efficiency).Furthermore, since the phosphor layer was no longer smooth, any lightrays that were internally reflected could find themselves rescattered toareas far from their point of creation, thus spoiling the resolution ofthe display.

[0019] Microcavity resonators, which can be incorporated in the presentinvention, have existed for some time and have recently been describedby H. Yokoyama, “Physics and Device Applications of OpticalMicrocavities” 256 Science 66 (1992) Microcavities are one example of ageneral structure that has the unique ability to control the decay rate,the directional characteristics and the frequency characteristics ofluminescence centers located within them. The changes in the opticalbehavior of the luminescence centers involve modification of thefundamental mechanisms of spontaneous and stimulated emission.Physically, such structures as microcavities are optical resonantcavities with dimensions ranging from less than one wavelength of lightup to tens of wavelengths. These have been typically formed as oneintegrated structure using thin-film technology. Microcavities involvingplanar, as well as hemispherical, reflectors have been constructed forlaser applications.

[0020] Resonant microcavities with semiconductor active layers, forexample silicon or GaAs, have been developed as semiconductor lasers andas light-emitting diodes (LEDs).

[0021] E. F. Schubert, et al, “Giant Enhancement of LuminescenceIntensity in Er-doped Si/SiO2 Resonant Microcavities” 61(12) Appl. Phys.Lett. 1381 (1992), describes a resonant microcavity with an Er dopedSiO₂ active layer. This device emits radiation in the infrared regionand is intended as a laser amplifier for fiber-optic communications.

[0022] The Schubert device, the semiconductor lasers and the LEDs arenot as suitable for use in luminescent displays for several reasons,They contain luminescent materials such as Si, GaAs, etc., in the activeregion which are suitable as laser media, but which are typicallyinefficient emitters of visible light and require excitation by theinjection of electrons. They also are designed with small planar surfaceareas that are inadequate for display purposes. Moreover, because of thedesign of these devices and the active materials used, they typicallycannot be excited efficiently with electron bombardment, an electricfield, or ultraviolet radiation. These excitation mechanisms are anessential part of the current display technologies.

[0023] Furthermore, the laser microcavity devices work above the laserthreshold, with the result that their response is inherently nonlinearnear this threshold and their brightness is limited to a narrow dynamicrange. Displays, conversely, require a wide dynamic range of brightness.Microcavity lasers utilize stimulated emission and not spontaneousemission. As a result, these devices produce highly coherent lightmaking these devices less suitable for use in displays. Highly coherentlight exhibits a phenomenon called speckle. When viewed by the eye,highly coherent light appears as a pattern of alternating bright anddark regions of various sizes. To produce clear, images, luminescentdisplays must produce incoherent light.

[0024] In addition, it is important to distinguish the resonantmicrocavity display from the laser CRT. This display is similar to a CRTand scans an electron beam to write the information to the luminescentscreen. However, the light is not produced by the spontaneous emissionof the phosphor, but by stimulated emission. The faceplate of the laserCRT is an electron beam pumped semiconductor laser. The active medium, asemiconductor, is placed between-two mirrors that form a laser cavity.The cavity structure is contained within the faceplate. When pumped witha sufficiently energetic electron beam, the device lases, producing ahighly energetic and directional light beam. Such a display is describedby A. S. Nasibov, et. al. in the article “Full Color TV projector basedon A₂B₆ electron-pumped semiconductor lasers”, J. Crystal Growth, 117,1040 (1992).

SUMMARY OF THE INVENTION

[0025] The subject invention, the Resonant Microcavity Display (RMD), isa luminescent display which offers the advantages of a thin-filmphosphor without exhibiting the light piping problem. This is because itemits light in a highly directional manner as a result of its geometry.

[0026] The resonant microcavity display is any structure that modifiesspontaneous emission properties of a phosphor contained within thestructure. The modification of spontaneous emission is obtained bychanging the optical mode amplitudes to the such a degree that thephosphor favorably emits into a relatively few optical modes. It is alsopossible to suppress emission in certain optical modes. Thismodification of mode amplitudes can be created, for example, by theformation of a standing wave electric field for each favored mode withinthe structure and locating the phosphor at the antinodes of thesestanding waves. It is essential that the standing waves havesubstantially modified electric field amplitudes relative to the thefield amplitudes generated without a cavity. Substantially modifiedrefers to changes by a factor of two or more in the field amplitudes.

[0027] In standing wave cavities, no enhancement can occur at the nodeof the electric field. However, a ring cavity design 320 such as thatshown in the downward-looking view of FIG. 1 supports a traveling wave322 in which the electric field amplitude is substantially modifiedthroughout the entire cavity. As a result, mode enhancement orsuppression can occur throughout the cavity. Compared to the standingwave cavity, more active medium 324 with modified light emission can beutilized for the same cavity volume.

[0028] One example of a resonant microcavity display is a microcavityresonator comprising a phosphor sandwiched between two reflectors, allof which are grown on a transparent rigid substrate. The width of theactive region is chosen such that a resonant standing wave, of thewavelength to be emitted, is produced between the two reflectors. In itssimplest form, a single coplanar microcavity, the two reflectors areparallel to each other and the plane of the active region is parallel tothe reflectors. Other geometries which produce standing waves ortraveling waves with an increased electric field amplitude, such ascombinations of planar microcavities, three-dimensional microcavities,confocal microcavities, hemispherical microcavities, or ring cavitiesare also possible. These other geometries are well-known, in the art ofdesigning cavities.

[0029] Another structure that favorably alters the spontaneous emissionproperties uses photonic band gap crystals. A photonic band gap crystalcan be formed from a monodispersed colloidal suspension. The structurescomprise periodic dielectric media to create a band gap of energy forwhich light cannot propagate within the structure. However, doping sucha structure with a material that has a resonance within the band gapwill create a high Q cavity. Such cavities can be one, two or threedimensional. The cavity generates a standing wave with an enhancedelectric field amplitude in the region of the dopant. In order to createa display, the photonic band gap crystals must be a phosphor. Henry O.Everitt describes photonic band gap crystals in “Applications ofPhotonic Band Gap Structures”, Optics & Photonics News, 20, (1992). FIG.2 is a side view of a resonant microcavity display 350 on a substrate352 using a photonic band gap crystal 354 as the entire cavitystructure.

[0030] Fabricating the RMD requires the use of a growth techniquecapable of controlling layer thickness or the spatial resolution of therefractive index to a precision of several nanometers. Such techniques,for example, include, but are not limited to, chemical vapor deposition(CVD) molecular beam epitaxy (MBE), atomic layer epitaxy (ALE), electronbeam evaporation, or sputtering. Fabricating the RMD may also employholographic photo-lithographic techniques. In this case, the Braggreflectors are created by exposing a suitable material to a holographicpattern thereby creating in the material alternating layers of high andlow refractive index regions. Such a technique is well known in the artof fabricating holographic diffraction gratings.

[0031] The substrate can be either a crystalline, polymer, or anamorphous solid. It can be made of any material that will allow theother regions to be grown on it. Suitable substrate materials may bechosen from a wide range of materials such as oxides, fluorides,aluminates, and silicates. The substrate material can also be fabricatedusing organic materials. The criteria involved in selecting a substratematerial include its thermal conductivity and its compatibility (bothphysical and chemical) with other materials forming the RMD.

[0032] The phosphor may be excited through several means, including:bombardment by externally generated electrons (cathodoluminescence),excitation by electrodes placed across the active layer to create anelectric field (electroluminescence), or excitation using photons(photoluminescence).

[0033] The present invention is distinguished from other microcavitydevices in part by the placing of a phosphor in the resonantmicrocavity. Phosphors are materials that exhibit superior visibleluminous efficiencies (where luminous efficiency, as used herein, isdefined as the ratio of light output in Watts over the power input inWatts). Typically, the luminous efficiencies of phosphors range between1% and 20%. These high efficiency materials are only classified asphosphors if the material efficiently generates luminescence whenexcited by electrons, electric fields, or light.

[0034] The active region may comprise a wide range of inorganicphosphors (e.g., sulfides, oxides, silicates, oxysulfides, andaluminates) most commonly activated with transition metals, rare earthsor color centers. In addition to inorganic phosphors, the active regionmay employ an organic phosphor such as tris (8-hydroxyquinoline)aluminum complex. The active region comprises phosphors typically in theform of single crystal films, polycrystalline films, amorphous films,thin powder layers, liquids, or some combination of the above. Aselection of phosphors that have found commercial applications, and fromwhich an application dependent phosphor can typically be selected foruse in the present invention, is documented in “Optical Characteristicsof Cathode Ray Tube Screens,” Electronic Industries AssociationPublication TEP 116.

[0035] The reflectors forming the resonant cavity consist of eithermetallic layers or Bragg reflectors. Bragg reflectors are dielectricreflectors formed from alternating layers of materials with differingindices of refraction. The simplest geometry for dielectric reflectorsconsists of one-quarter wavelength thick layers of a low refractiveindex material, such as a fluoride or certain oxides, alternating withone-quarter wavelength thick layers of a high refractive index material,such as a sulfide, selenide, nitride, or certain oxides. The dielectricreflectors can also be fabricated using organic materials. Mirrors canalso be formed using photonic band gap crystals. Any incident light withan energy within the band gap will be reflected by the structure. FIG. 3shows a side view of an illustrative embodiment of a resonantmicrocavity display 340 on a substrate 342 in which an active layer 346is sandwiched between two mirrors 344, 348 comprising photonic band gapcrystals.

[0036] In current display applications, only one side of the screen isviewed. In the case of a microcavity, the design requires the use ofdifferent reflectors in order for most of the light to be projectedtowards the viewer. In the case of the simple coplanar microcavity, thisasymmetry is obtained by having one of the two reflectors besubstantially wholly reflective, meaning that it reflects most of thelight impinging on it. The other reflector (opposite to thesubstantially wholly-reflective reflector) is partially reflective,meaning that it does not reflect as high of a percentage of impinginglight as the wholly-reflective reflector and allows some of the light topass through it. Because of the difference in reflectance of the tworeflectors, virtually all of the light produced in the active regionescapes through the partially-reflective reflector along the axis normalto the plane of the device.

[0037] In the case of a microcavity structure, the dimensions depend onthe natural spontaneous emission spectrum of the phosphor being used, asobserved outside of a cavity. If the spectrum covers a broad range ofvisible wavelengths it is possible to choose an appropriate part of thespectrum (i.e., one that matches an industry standard chromaticity) andconstruct the microcavity with a matching resonance. The finalchromaticity of the RMD will correspond to the cavity resonance and willbe different from the natural chromaticity of the phosphor outside ofthe microcavity. Conversely, if the phosphor's natural spontaneousemission spectrum covers only a narrow range of visible wavelengths, thedimensions would be chosen so that the cavity resonance would match oneof the phosphor's emission bands.

[0038] The RMD has a highly directional light output similar to those ofa projector or a flashlight and, as a result, RMDs can be constructed toavoid light piping. This allows highly efficient coupling to otherdevices. RMD's also have a high external efficiency, approaching 100%.Since RMDs incorporate films, RMDs permit the design of efficientthermal conduction of the heat generated in the active layer. Thisfeature combined with the ability to reduce the phosphor decay timeallow RMDs to utilize intense excitation. As a result of the above, RMDsare especially suitable for use in projection displays.

[0039] It is therefore an object of this invention to provide aluminescent display that does not exhibit the problem of light piping.

[0040] It is a further object of this invention to provide a luminescentdisplay with highly efficient heat transfer properties.

[0041] It is a further object of this invention to provide a luminescentdisplay with a high external efficiency.

[0042] It is a further object of this invention to provide a luminescentdisplay capable of high resolution.

[0043] It is a further object of this invention to provide a luminescentdisplay which produces a highly directional output.

[0044] It is a further object of this invention to provide a luminescentdisplay in which the chromaticity of the emitted light can be accuratelycontrolled irrespective of the nature of the phosphor used.

[0045] It is a further object of this invention to provide a luminescentdisplay wherein the phosphor used can be chosen to optimize the displaywith respect to properties other than chromaticity.

[0046] It is a further object of this invention to provide a luminescentdisplay wherein the decay time of the activator can be tailored for thespecific display application.

[0047] It is a further object of this invention to provide a luminescentdisplay which can be heavily loaded by the excitation source withoutsaturating the phosphor due to overheating.

[0048] Other objects and advantages of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of the illustrated embodiments, when read in light of theaccompanying drawings.

[0049] Througout this specification, published articles are cited forbackground purposes. These articles are hereby incorporated by referenceinto this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a top sectional view of a traveling wave cavity in oneillustrative embodiment of a resonant microcavity display in accordancewith the invention.

[0051]FIG. 2 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention using aphotonic band gap crystal as a resonant microcavity display.

[0052]FIG. 3 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention usingphotonic band gap crystals as mirrors.

[0053]FIG. 4 is a perspective illustration of an illustrative embodimentof a resonant microcavity display in accordance with the inventionemploying a planar mirror resonator.

[0054]FIG. 5 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention employinga confocal resonator.

[0055]FIG. 6 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention usingmultiple cavity structures.

[0056]FIG. 7 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the inventionincorporating an integral optical element.

[0057]FIG. 8 is a perspective view of one illustrative embodiment of aresonant microcavity display in accordance with the invention employingcathodoluminescent excitation.

[0058]FIG. 9 is a side sectional view of an illustrative embodiment of aresonant microcavity display in accordance with the invention as itwould be used in a cathode ray tube.

[0059]FIG. 10 is a side sectional view of an illustrative experimentalembodiment of a resonant microcavity display in accordance with theinvention designed to emit light through its front reflector with awavelength of 530 nanometers.

[0060]FIG. 11 is a graph relating the reflectance of the resonantmicrocavity display of FIG. 10 as a function of the wavelength of theincident light.

[0061]FIG. 12 is a side sectional view of a direct view color televisionemploying a resonant microcavity display in accordance with theinvention.

[0062]FIG. 13a is a perspective illustration of an array of pixel-sizedmicrocavities as used in a color television in accordance with theinvention.

[0063]FIG. 13b is an illustration of a front view of an array ofpixel-sized microcavities as used in a color television in accordancewith the invention. The front view shown in FIG. 13b corresponds to aview from the top of FIG. 13a.

[0064]FIG. 14 is a side sectional view of an illustrative embodiment ofa resonant microcavity display in accordance with the inventionincorporated in a vacuum fluorescent display.

[0065]FIG. 15 is a side sectional view of an illustrative embodiment ofa resonant microcavity display in accordance with the invention using anarray of high voltage field emission devices for excitation of itsactive layer.

[0066]FIG. 16 is a side sectional view of an illustrative embodiment ofa resonant microcavity display in accordance with the invention using alow voltage field emission material for excitation of the active layer.

[0067]FIG. 17 is a schematic illustration of the standing wave electricfield in one illustrative embodiment of a resonant microcavity displayin accordance with the invention.

[0068]FIG. 18 is a perspective drawing of an illustrative embodiment ofa resonant microcavity display in accordance with the invention excitedby an electric field.

[0069]FIG. 19 is a perspective drawing of an illustrative embodimentresonant microcavity display in accordance with the invention excitedwith ultra-violet light.

[0070]FIG. 20 is a side sectional view of an illustrative embodiment ofa transparent resonant microcavity display in accordance with theinvention.

[0071]FIG. 21 is a schematic illustration of an illustrative embodimentof a resonant microcavity display in accordance with the inventionemploying a laser for excitation.

[0072]FIG. 22 is a schematic illustration of an illustrative embodimentof a tunable resonant microcavity display in accordance with theinvention.

[0073]FIG. 23 is a schematic illustration of an illustrative embodimentof a resonant microcavity display in accordance with the invention usedas a light source for a liquid crystal display light valve application.

DETAILED DESCRIPTION OF THE INVENTION

[0074] The present invention employs quantum electrodynamic (QED) theoryto enhance the properties of the light emitted from phosphor basedluminescence displays. The performance of a given display applicationdepends on properties of the emitted light such as the chromaticity,direction, and flux. These properties can be optimized by employing theprinciples of QED theory in the design of microcavities so as to controlthe spontaneous emission characteristics of the phosphor activator foreach specific display application.

[0075] As seen in FIG. 4, one example of the present invention 10comprises a phosphor embedded in a resonant microcavity 20 grown on asubstrate 25. The microcavity 20 further comprises a front reflector 30,a phosphor-based active region 50, and a back reflector 60. The activeregion 50 is disposed between two reflectors 30 and 60. The structuremay comprise a variety of materials and may employ a variety ofresonator designs. FIG. 4 illustrates a planar mirror design, whereasFIG. 5 illustrates the present invention configured in a confocal mirrordesign. The confocal design has the advantage of having an inherentlyhigher cavity quality factor (Q).

[0076] More complex cavity designs involve stacking multiplemicrocavities. This design is similar to the standard method for forminginterference devices which typically consist of 2 or more stackedcavities where each cavity is separated by a coupling layer. Suchstructures are used in the fabrication of, for example, bandpass opticalfilters, narrow band optical reflectors and long wavelength or shortwavelength cutoff filters.

[0077] The invention can only be completely understood by employingquantum electrodynamic (QED) theory as applied to a cavity. Cavity QEDcalculations allow one to determine the following parameters for a givendegree of activator excitation and activator concentration: the amountof light emitted from the microcavity; the angular spread of the lightemitted; and the color of the light emitted.

[0078] The calculation begins by determining the nature of theelectromagnetic field inside and outside of the cavity. This fieldcalculation uses Maxwell's equations with the boundary conditionsimposed by the microcavity. Applying Fourier analysis, the netelectromagnetic field is broken down into its fundamental constituents,the optical modes.

[0079] An optical mode is a field with a characteristic frequency,direction and polarization. The square of the field intensitycorresponds to the actual amount of light. One must select from thisfield distribution those optical modes that correspond to useful light.For a display, useful light is defined as any light emitted from thecavity within a certain predetermined angular spatial distribution andpredetermined frequency spread.

[0080] The next step is to calculate the amount of light emitted by eachactivator. This calculation begins by determining the radiative decayrate of each activator for each possible optical mode. The radiativedecay consists of a spontaneous emission rate and a stimulated emissionrate. The resonant microcavity display, however, only operatessatisfactorily as a display when there is no stimulated emission (i.e.,constructing a microcavity to operate as a laser would preclude using itas a display). The degree of excitation, the type and concentration ofthe activators and the resonator design determines when stimulatedemission is an issue.

[0081] The spontaneous emission rate is determined by using QED theoryto calculate the probability that a single excited activator will decayinto a specific optical mode. This calculation must use the fieldstrength appropriate for the location of the activator in the cavity.The magnitude of the standing or traveling wave within the cavity mayhave different values throughout the phosphor layer. In addition, acertain probability exists that each excited activator will decaywithout emitting light. To calculate this non-radiative rate, one mustconsider cavity QED effects as they apply to the physical mechanismresponsible for the non-radiative decay.

[0082] For a given excitation level, one can now calculate the amount ofspontaneous emitted light for each activator. The ratio of thespontaneous rate to the sum of the radiative and non-radiative ratesyields the percentage of excitation that will produce light. The amountof useful light is then determined by calculating the amount of thespontaneous emission in the desired optical modes. This calculation isperformed for each activator. Finally, the sum of all the activatorcontributions yields the display intensity of the RMD.

[0083] The properties of the RMD that can be controlled include thechromaticity, the directionality of the display, the luminous efficiencyand the maximum light output of the display. These properties are tunedaccording to the requirements of the specific luminescent screenapplication. The parameters that must be considered for optimization arethe microcavity Q, the microcavity resonance frequency, the asymmetry ofthe reflectors, the resonator design (i.e., planar, confocal, multiplecavity, etc.), the phosphor, the thickness of the phosphor layer, thesurface area of the microcavity and the excitation source. Theseparameters cannot be optimized separately; each affects the otheradjustable properties of the display.

[0084] The performance of the resonant microcavity can be described bythe Q of the cavity. The Q of the cavity is given by the microcavitycenter frequency divided by the linewidth of the microcavity resonance:$Q = \frac{v}{\Delta \quad v}$

[0085] where ν is the microcavity resonance frequency and Δν is thelinewidth of the cavity resonance. The cavity Q is determined primarilyby the reflectance of the reflectors, the resonator design, theasymmetry in the reflectance and any imperfections in the cavity. Theseimperfections typically result from defects in the structure of theresonant microcavity which scatter light out of the cavity in anon-useful manner. The Q can be measured empirically using an opticalspectrometer.

[0086] As the cavity Q increases, the display brightness and efficiencyincreases. In addition, the angular spread of the light decreases andthe linewidth shrinks altering the chromaticity. Note that as thespatial distribution of the light narrows, the amount of light incertain regions decreases. Depending on the display application, thiseffect may or may not be desirable. For the range of the current displayapplications, the engineered cavity Q will typically vary between 10 and10,000. The above effects can be determined experimentally by measuringthe light intensity as a function of solid angle for resonantmicrocavities with different Q values. Using this data, one can predictthe required Q for a given application.

[0087] For most current applications, only one side of the luminescencescreen is viewed. In these applications one should choose reflectorswith different reflectivities such that the display preferentiallyforces the light out the cavity towards the viewer.

[0088] The resonator design directly affects the Q and mode volume. Thelatter term describes the actual volume of the activator layer that isparticipating in producing useful light. This volume is related to thespatial distribution of the electromagnetic field within the activatorlayer. The design of the resonator will also determine the spatialdistribution of useful light. Due to the relatively straightforwardconstruction, the simplest design is a planar resonator. However, otherresonator structures which produce standing waves or traveling waveswith an enhanced electric field intensity in a phosphor material may beuseful. In particular, multiple planar microcavities may be combined toallow for a larger active region or to achieve greater control over theallowed emission than can be achieved with a single cavity.

[0089]FIG. 6 provides one illustrative design for a 3 cavity resonantmicrocavity 200. In this example, each cavity 201A, 201B, 201C comprisesdielectric reflectors 202, 206. The dielectric reflectors 202, 206 areseparated by a half-wavelength coupling layer 204 in cavities 201A and201B, while adjacent cavities are separated from each other by ahalf-wavelength spacer 208. The phosphor material 209 is also ahalf-wavelength thick and is located within the lowest cavity, takingthe place of a half-wavelength coupling layer in cavity 201C. Thedistances specified are optical thicknesses, i.e. the index ofrefraction multiplied by the physical thickness of the layer.

[0090] As already discussed in the case of the planar geometry, thereexists an entire set of parameters to consider including the individualmirror reflectances and individual cavity Q's. In addition, one mustalso determine the cavity spacing, coupling layer, and the location ofthe phosphor material. The exact specifications will depend on thespecific display requirements.

[0091] A primary design specification of the RMD is the chromaticity ofthe emitted light. The center frequency and linewidth of the cavity mustbe engineered so that the RMD displays this color of light.

[0092] Once these parameters are selected, the phosphor must beselected. The phosphor will need to have a natural luminescenceresonance that overlaps the cavity resonance. As the resonance narrowsand the overlap increases, the display efficiency and brightnessincrease. A compromise between chromaticity and other parameters may berequired to optimize a display for a specific application.

[0093] The intensity of light emitted by the phosphor is related to theactivator concentration: as the concentration increases, the intensityof emitted light increases. The activator concentration, however, isoften limited by non-radiative energy transfer between activators thatquenches luminescence. These quenching effects are concentrationdependent. The quenching concentration varies from phosphor to phosphor,depending on the magnitude of various energy transfer parameters betweenactivators. Cavity QED theory predicts that there is an effect on theseparameters since they relate to spontaneous emission characteristics.Thus, another potential advantage of the RMD is that energy transferbetween activators may be suppressed and phosphors could contain higherconcentrations of activators than was previously possible, withoutlosing efficiency. In addition, phosphors can simultaneously emitseveral wavelengths corresponding to different optical transitionswithin the material. However, only one of these transitions typicallygenerates the useful light of the display. A microcavity can be designedto enhance this useful transition while inhibiting the non-usefultransition(s). This suppression will increase the efficiency of thedisplay. The ability of a structure to inhibit spontaneous emission andenergy transfer processes has been described by G. Kurizki and A. Z.Genack in “Suppression of Molecular Interactions in Periodic DielectricStructures”, Phy. Rev. Let. 61, 2269 (1988).

[0094] The display properties also depend upon the thickness of theactive region. Depending on the cavity design, there may be severalactive region thicknesses that produce a predetermined frequency. Therange of thickness depends on the mirror construction. As the thicknessincreases, the number of potentially excited activators increases. Withsufficient excitation energy, the total luminescence can be increasedwith a wider active region. However, the thickness may alter the spatialdistribution in a highly complex manner. In the case of a simplecoplanar microcavity, the angular spread of the light changes, withadditional regions of high intensity appearing at angles that are notnormal to the plane of the microcavity. More complex multiple cavitydesigns allow a greater degree of control over the directionality of thedisplay.

[0095] Another key parameter in the resonant microcavity design is thearea of the emitting surface. Some applications will require onelarge-area surface for the production of monochromatic light, whileother designs will need pixel-sized cavities capable of producing red,green and blue light. The size of the pixel will be determined by theresolution requirements of the display.

[0096] One other important parameter is the excitation source andintensity. The display application will dictate the excitation source.The decision process in selecting the phosphor must also consider theefficiency of converting the excitation energy into useful luminescence.This efficiency is well documented for the registered phosphors, but caneasily be experimentally determined. The intensity of the source willprimarily change the brightness.

[0097] It should be noted that in considering the above designparameters, the light properties of the display must not reach thedegree of coherence associated with a laser. To avoid this problem,particular attention must be paid to the cavity Q, the activatorconcentration and the excitation intensity.

[0098] The RMD design lends itself to the incorporation of an opticalelement 382, such as a lens or a diffuser, fabricated within or on topof the substrate 384 of a resonant microcavity 386, as shown in FIG. 7.For example, a lens would be useful to modify the angular distributionof the light output produced by the structure and thereby generate therequired distribution. The lens may be formed using photo-etchingmethods, which is well known in the art of miniature semiconductorlasers. Another method would employ the controlled placement ofimpurities to change the local refractive index. This method is used toconstruct gradient refractive index lenses which are commonly used infiber optics.

[0099] Using such a lens adds another parameter that must be consideredin the optimization of the display. However, such a lens enables one tomaximize the output of the resonant microcavity without having toconsider the required light distribution. For example, such a lens wouldeliminate or reduce the demands for the complex lens design currentlyrequired in the projection CRT display applications.

[0100] Similarly, a diffuser can be used to precisely control theangular spread of the light and thereby the field of view of thedisplay. With the ability to control the light distribution independentof the microcavity, the spontaneous emission properties of the phosphorcan be maximized without having to consider the required lightdistribution. A diffuser can be made using holographic techniques, ruledgrating techniquess, introduction of internal scattering centers, orprecisely controlled surface roughening.

[0101] The RMD can be embodied using cathodoluminescence which resultsfrom an electron beam bombardment of the phosphor. One example of adevice which employs cathodoluminescence is a projection television.This application requires the highest intensities possible because itrequires a wide viewing area and uses a light dispersing screen. In thisapplication, the resonant microcavity display is incorporated in a CRT.

[0102] Full color projection televisions require three separate CRT's:one for each primary color. In this application, the RMD is superior toconventional methods because it allows intense excitation loading of thephosphor, highly directional output, controlled chromaticity, and highexternal efficiency. Therefore the RMD allows the use of relativelycompact CRT's while maintaining high luminescence.

[0103] In the case of a resonant microcavity display incorporated in aCRT, the phosphor is excited by electrons emitted from the electron gun,accelerated to a speed such that most of them will penetrate theresonant microcavity to the depth of the phosphor. The high energyelectrons excite electrons in the phosphor from the valence band intothe conduction band. This additional energy is trapped at the impurity.The impurity then relaxes by emitting visible light.

[0104] In the case of a simple coplanar microcavity, the reflectors canhe either dielectric or metallic. The back reflector has a higherreflectivity than the front reflector, so that light, emitted by thephosphor, exits the cavity through the front reflector, perpendicular tothe plane of the thin film device. The microcavity Q and the asymmetryin the reflectance determines the percentage of light that exits theresonator through the front reflector.

[0105] In the case of the simple coplanar microcavity, the width of theactive region affects the directionality of the light and is chosen sothat its optical path length, i.e., the product of the distance betweenthe back reflector and the front reflector and the index of refractionof the phosphor material, equals an integer multiple of the desiredwavelength divided by 2 or 4 depending on the index of the adjacentlayers. These dimensions ensure that a standing wave builds up betweenthe back-reflector and the front reflector. The wavelength of theemitted light is determined by the resonant wavelength of themicrocavity.

[0106] A dielectric, or Bragg, reflector consists of alternating layersof material with high and low indices of refraction. The number oflayers determines the reflectivity of the reflector. The reflectivity(R) of the reflectors can be calculated using the following equation:$R = \frac{1( \frac{n_{H}}{n_{L}} )^{N - 1} \times \frac{n_{H}^{2}}{n_{2}}}{1 + {( \frac{n_{H}}{n_{L}} )^{N - 1} \times \frac{n_{H}^{2}}{n_{2}}}}$

[0107] where n_(H) and n_(L) are the refractive index of the high andlow index of refraction materials, respectively; n_(s) is the index ofrefraction of the substrate and N is the total number of layers in thestack. This equation is valid for normal incidence. The width of eachlayer is equal to an odd integer multiplied by the desired wavelength oflight to be emitted divided by the quantity 4 times the index ofrefraction of the material used in tie layer. An alternate design usesholographic techniques to form the reflectors. In this case, the mirroris formed from one material with a continuously varying refractiveindex. Photo-lithography would be used to fabricate the mirrors.

[0108] The Q of the cavity can be calculated once the reflectivity isdetermined for the reflectors. In the case of the simple coplanarmicrocavity, the equation that relates Q to reflectivity is given by:$Q = \frac{2\pi \quad n\quad v}{c( {\alpha - {\frac{1}{l}( {m\sqrt{R_{1}R_{2}}} )}} }$

[0109] where ν is the microcavity resonance frequency, n is the index ofrefraction of the phosphor, α is the average distributed loss constant,l is the width of the activator layer, R₁ is the reflectance of thefront mirror and R₂ is the reflectance of the back mirror. The constantα is needed to account for the non-ideal behavior of the cavity thatresults from imperfections and spurious absorption.

[0110] The parameters chosen to optimize this display depend on therequired brightness of the display and the required directionality ofthe light output. In the typical projection television application, thedisplay should be highly directional and bright. For each color, thecavity Q can be optimized empirically by measuring the total intensityemitted in the useful direction as a function of the electron beamcurrent. This efficiency measurement is common in the television designart.

[0111]FIG. 8 shows one illustrative embodiment designed forcathodoluminescence, the simple planar resonant microcavity, The subjectinvention 10 comprises a resonant microcavity 20 grown on a rigidtransparent substrate 25. A layer of aluminum 80 is disposed next to themicrocavity 20 to channel off electrons deposited by the electron beamand to provide an additional reflective surface. The resonantmicrocavity 20 is grown onto the substrate 25 using molecular beamepitaxy (MBE) or any suitable method of solid-state fabrication. Somemethods of growth known to the art (e.g., LPE at its current level ofdevelopment) are not suitable because they cannot be controlled with theprecision necessary to grow a correctly sized microcavity. The activeregion 50 is excited by electrons from an electron beam 54 enteringthrough the aluminum layer 80 and back reflector 60. The light 58created in the active region exits through the front reflector 30 andthe substrate 25.

[0112] As seen in FIG. 9, this embodiment can be embodied in a cathoderay tube (CRT) 100 comprising a glass vacuum tube 105 enclosing anelectron gun (which is a means to generate an electron beam) 110 aimedat a flat viewing surface 115 and distal from the electron gun 110; anda phosphor-based resonant microcavity 20 disposed parallel to the flatviewing surface 115 inside the vacuum tube 105. This embodiment isconfigured to produce monochromatic light.

[0113] As shown in FIG. 10, an experimental embodiment designed to emitlight through the front reflector with a wavelength of 530 nanometers,the material used in the active region 50 is zinc sulfide (ZnS) dopedwith manganese (Mn) at a dopant concentration of 2%. The thickness ofthe active region 50 is 110 nanometers and the phosphor has an index ofrefraction of n=2.4.

[0114] In the front reflector 30, the material used in the layers with arelatively high index of refraction 32, 36, 40 and 44 is ZnS, and thematerial used in layers with a relatively low index of refraction 34,38, 42 and 46 is calcium fluoride (CaF₂). In the back reflector 60, thematerial used in the layers with a relatively low index of refraction62, 66, 70, 74, 77, and 79 is CaF₂, and the material used in the layerswith a relatively high index of refraction 64, 68, 72, 76, and 78 isZnS. All of the high-index ZnS layers are 55 nanometers thick with anindex of refraction of n=2.4. All of the low-index CaF₂ layers are 95nanometers thick with an index of refraction of n=1.4.

[0115] The substrate 25 is made of CaF₂. It is 2 millimeters thick andhas an index of refraction of n=1.4. The aluminum layer 80 is 50nanometers thick.

[0116] The microcavity 20 is grown on the substrate 25 using MBE and thealuminum layer 80 is deposited using vapor-phase deposition.

[0117] The front reflector has a reflectivity of R=97.5% with 8 layersand the back reflector has a reflectivity R=99.9% with 12 layersincluding the aluminum layer. Because the back reflector is morereflective than the front mirror almost all of light produced in thecavity exits through the front reflector.

[0118] As shown in FIG. 11, the reflectance of the RMD is a function ofthe wavelength of the incident light. At the resonance wavelength of 530nm, the reflectance dips to roughly 86%—indicating that the RMD willtransmit this wavelength. At all other wavelengths the reflectance isnear 100%—indicating that the RMD will not transmit light atnon-resonance wavelengths. This reflectance behavior is due to the factthat the cavity can only support a standing wave of a wavelength equalto the resonance wavelength of the cavity.

[0119] In another embodiment, the RMD can be used in a CRT as a directview television. FIG. 12 depicts a direct view color television. The CRT120 is similar to the one described in the projection televisionembodiment, except that it has three electron guns, 122, 124 and 126 onefor each primary color. Each of the electron guns produces a separateelectron beam, 130, 132 and 134, corresponding to the desired intensityof each color. The electron beams excite a screen 140 on the viewingsurface of the CRT.

[0120] As seen in FIG. 13a, the screen 140 comprises of an array ofpixel-sized microcavities 20. The array contains microcavities designedto produce red light 142, green light 144 and blue light 146. Thered-light pixels are excited by the “red” electron beam 130, thegreen-light pixels are excited by the “green” electron beam 132, and theblue-light pixels are excited by the “blue” electron beam 134. FIG. 13bshows a front view of the array of pixels and the arrangement of colors.The design of color displays with separate color pixels is well known inthe art.

[0121] In this embodiment, the light emanating from the pixel producesthe required angular distribution. One could also envision an embodimentin which a lens is used to achieve this display requirement allowing forthe maximum efficiency to be produced by the resonant microcavity. Therequired angular distribution can also be obtained using a diffuser suchas a holographic optical element.

[0122] The construction of the pixel is fundamentally the same as thatdescribed in the embodiment for a projection television. The primarydifference is the size of the surface area and the angular spread oflight required. In this case, the surface area is determined not bybrightness, but by the resolution required by the application. Highdefinition television, medical and military applications typicallyrequire the pixel size to be smaller than 25 microns. This requirementis difficult to achieve using current technologies, but can be easilyachieved using an RMD.

[0123] With the resolution and angular distribution specified, theresonant microcavity display must be optimized for each color. Thisoptimization will use the above-described empirical method of measuringthe total light produced versus beam current. The restrictions of thedesign due to the specification mean that obtaining the maximum lightoutput is primarily a function of the phosphor activator. In theembodiment in which a lens is placed outside the cavity, one has muchmore freedom in engineering the cavity. Without the restriction on theangular distribution, the cavity Q can be easily tailored.

[0124] In another embodiment using electrons that excite the activelayer, the resonant microcavity 217 can be incorporated in a vacuumfluorescent display 210, as shown in FIG. 14. Display 210 comprisesindividual pixels which are typically combined to form low resolution,compact information displays and extremely large displays.

[0125] A vacuum fluorescent display generally comprises an array ofcathodes 226, a control grid 224 and phosphor coated anodes,corresponding to anodes such as anode 214 shown in FIG. 14. (Anode 214shown in FIG. 14 differs from a conventional anode as described below.)Electrons are first generated by the hot filaments that form the cathodearray 226. A positive voltage is applied between the cathode array 226and anodes 214. When the control grid voltage is on, the electrons areaccelerated by the positive potential towards the phosphor layer whichis deposited, in a conventional vacuum fluorescent display, on top ofthe anodes. The remainder of the display conventionally comprises aglass faceplate 212, glass backplate 228, and a glass frit seal 222,containing a vacuum for the control wire grid 224 and filament cathodes226.

[0126] A resonant microcavity structure may be used to improve theperformance of this type of display. One possible illustrativeembodiment is depicted in FIG. 14. The resonant microcavity structure217 comprising an active layer 218 sandwiched between a pair ofdielectric mirrors 216, 220 disposed between the control wire grid 224and the anode 214 replaces the powder phosphor that is conventionallydeposited on anodes such as anode 214.

[0127] For small scale monochromatic displays, one resonant microcavity217 would be used and the pixels would be determined by the control grid224 and cathode 226 arrangement. If full color is required, a resonantmicrocavity 217 would be required for each primary color. An efficientlayout would comprise alternating stripes of microcavities 217 withseparate stripes for each color. In large screen applications, eachpixel would incorporate one resonant microcavity designed for a specificcolor. The array would then comprise a triad of red, green and bluepixels.

[0128] As discussed above for the two CRT embodiments, the parameterssuch as directionality, brightness, color and the microcavity structureapply for the vacuum fluorescent display. The design considerations andmethodology required for optimizing the display are also the same. Forexample, since this display is a direct view type, with light emissiondirected towards a viewer in the direction indicated by arrow A, thedivergence of the emitted light would be tailored for the viewerdistance and required viewing angle. Incorporation of lenses anddiffusers must be considered. The design of vacuum fluorescent displaysfor specific applications is well known in the art.

[0129] In another embodiment using excitation by electrons, the resonantmicrocavity can be incorporated into field emission displays for bothprojection and direct view applications. This display operates on theprinciple of electrons tunneling from a microscopic tip or a microscopicregion of a low work function material. The electrons are thenaccelerated via a positive potential and penetrate an adjacent phosphorlayer. Typically there is a evacuated region between the tips and thephosphor, but in some applications the phosphor may be grown directly ontop of the emitting surface.

[0130] These displays may operate in both a high voltage and low voltagemode. In the high voltage application, typically above 500 volts, anemitter array would be assembled behind each microcavity. The displaycould consist of one microcavity that is the size of an entire displayto generate monochromatic light or the display could consist of pixelsize microcavities suitable for producing color images. In thesestructures, the voltage must be sufficiently high that the electrons canpass through the bottom mirror of the RMD into the active layer tostimulate the phosphor.

[0131]FIG. 15 provides one illustrative embodiment of a monochromaticfield emission display 230 incorporating a resonant microcavity 239comprising an active layer 236 between mirrors 234 and 238. When apositive high voltage is applied between the anode 240 and cathode 246,the electrons are generated by the field emissive material 244, which issealed within an evacuated region 242 by seals 248. The electrons arethen accelerated through the evacuated region 242, penetrate theresonant microcavity 239 and excite the active layer 236. The aluminumlayer 240 is approximately 50 nanometer thick and conducts the electronsto ground.

[0132] In the low voltage application, the field emissive material mustbe located inside the RMD due to the limited penetration depth of thelow energy electrons. Suitable emissive materials must have a low workfunction so that a low voltage applied to the material will inducesufficient numbers of electrons to be emitted. In this application, alow voltage is applied across the resonant microcavity and induceselectrons to tunnel from the electron emissive material into thephosphor and excite the activators. Under the influence of the appliedfield, the electrons travel through the phosphor and then into anothermaterial which conducts the electrons to ground.

[0133] In one illustrative embodiment of a low voltage field emissiondisplay 250 illustrated in FIG. 16, the resonant microcavity 253 (whichis deposited on substrate 252) would comprise an oriented diamond filmlayer 256 that is deposited on one side of the phosphor layer 258.Another conductive film layer 260 similar to diamond film layer 256would be deposited on the opposite side of phosphor layer 258 to conductthe emitted electrons to ground. A low voltage potential would beapplied between conductive layers 260 and 256. Reflectors 254, 262 aredisposed outside the sandwich-like structure formed by conductive layers256, 260 and phosphor layer 258. This embodiment is depicted in FIG. 16.

[0134] In the case of the simple coplanar microcavity, the key designspecification in all the display applications is locating the activelayer at or near the antinode of the electric field inside the cavity.In the low voltage field emission display, this specification iscritical given the thickness of the active layer. The basic structure ofa phosphor layer sandwiched between two emissive layers can be repeated,provided that the phosphor material is located at or near an antinode.An illustration of a standing wave field in one illustrative embodimentof a resonant microcavity display 300 is shown in FIG. 17. Microcavitydisplay 300 comprises a substrate 302, a pair of mirrors 304, and anactive layer 306. The electric field amplitude 310 in active layer 306is shown schematically. A node 311 and an antinode 312 are shown.

[0135] The design issues one must consider are fundamentally the same ashas been discussed in the other display applications. However, the indexof the electron emissive material must now be a major factor in thedesign of the cavity. An additional concern is the choice of materialfor the specific applied voltage range.

[0136] In addition, the RMD can be embodied in an electroluminescentdisplay. In this display application, a RMD is sandwiched between twoconductors. A voltage signal is applied to the conductors and therebyinduces what is termed thin film electroluminescence (TFEL). An array ofpixel-size elements is constructed to form a luminescent screen creatinga TFEL flat panel display.

[0137] This embodiment would comprise an array of pixels, where eachpixel would be an electrically activated microcavity. FIG. 18 shows onepixel in the array 160. The pixel comprises a visibly transparentsubstrate 162, a layer of Indium doped Tin Oxide (ITO) 164 (atransparent metal) acts as ground, and a resonant microcavity 166. Theresonant microcavity 166 comprises a front reflector 168, aphosphor-based active region 170 and a back reflector 172. Disposed nextto the back reflector 172 is an aluminum layer 174, which is depositedon each microcavity in such manner that each cavity is electricallyisolated.

[0138] This display would be excited by applying a voltage to thealuminum layer 174 of the pixel microcavity 166. The addressing ofpixels is common in the art of flat panel display design.

[0139] This display would be optimized by measuring the amount of usefullight emitted versus the electric field intensity, Particular attentionmust be paid to the phosphor selected since (in this embodiment) theelectroluminescence efficiency is important.

[0140] Also, the RMD could be embodied as an array of pixels in a flatpanel display which uses ultra-violet light to excite the phosphor. Asseen in FIG. 19, each pixel 180 would comprise a plasma discharge lamp182 that generates ultra-violet light which passes through a backreflector 184 and excites the active region 186 (i.e., the phosphor).The emitted light then passes out of the display through the frontreflector 183 and the substrate 190.

[0141] The RMD concept can also be used to fabricate a transparentdirect view flat panel display. This display is visibly transparentexcept at the specific resonant wavelengths of the microcavities thatare used in the display. Both monochrome and full color displays arepossible. For example, to create a full color display, one could chosethe three wavelengths that correspond to the three fully saturatedcolors specified by the international CIE color standard for red, greenand blue.

[0142] The transparent property is created by fabricating resonantmicrocavities that use reflectors that only function as high efficiencymirrors within a narrow wavelength bandwidth, typically one nanometer orless. Outside this region, the reflectors transmit nearly 100% and thusthe RMD appears transparent to the eye. Such narrow band reflectors canbe best built using a multiple cavity structure employing dielectricmirrors.

[0143] In one illustrative embodiment shown in FIG. 20, a flat paneldisplay would consist of an array of pixel size RMDs 500 excited by anelectric field. Two transparent electrodes 504, 514 must be connected toeither side of each microcavity 506 and could be best fabricated usingIndium Tin Oxide (ITO). Microcavity 506 itself would comprise an activelayer 510 between mirrors 508, 512.

[0144] In addition to creating a transparent display, the same reflectorstructure can be used to create a high contrast display. In thisembodiment, the rear surface is made opaque by another opaque layer (notshown) or by replacing ITO layer 514 with an opaque conductor. Externalambient light would be transmitted through the display and then absorbedby the rear layer. The reflection from the front surface would beminimized because of the high transmission properties of the reflectorsoutside the resonance wavelengths. Such a display can be made to havevery high contrast ratios on the order of a 100 or greater. These directview displays can use any of the three excitation sources.

[0145] The use of organic material permits the construction of a RMD outof flexible materials such as plastics.

[0146] The resonant microcavity display can also be excited using laserlight. Laser light results from stimulated emission processes and isdistinguished from spontaneous emitted light by the high degree ofspatial and/or temporal phase coherence. The laser light would be chosento have a wavelength that is absorbed by the phosphor. The cavitystructure must be designed to pass the laser wavelength. In oneembodiment shown in FIG. 21, a laser 412 would be scanned horizontallyand vertically across a luminescent screen 401 in a manner similar tothe electron beam in a cathode ray tube. The steering of beam 410 istypically accomplished by rotating mirrors and acoustic opticmodulators. The ability to write sequential information with lasers iswell known in the art. Luminescent screen 401 itself comprises substrate402 and microcavity 403, including mirrors 404, 408, and active layer406.

[0147] The RMD could also be used in a reverse configuration to absorblight and generate an electric signal. The physics that yields theenhanced emission of light demonstrated in the above display alsoproduces enhanced absorption. The light energy has to be converted intoelectric energy.

[0148] Another application of the resonant microcavity using itsproperty of enhanced absorption is in field of photography. In thisapplication, the film would comprise resonant microcavities in which theactive layer includes a photosensitive material. As a result, this filmwould absorb only at certain wavelengths corresponding to the threeprimaries. Since the amount of absorption can be precisely controlled,the film would be capable of extremely accurate color reproduction.Information could also be recorded by deriving an electrical signal fromthe photosensitive material within the microcavity. The general designwould be similar to digital cameras employing charge coupled detectors.

[0149] The unique ability of an RMD to influence the emissioncharacteristics may also be used in memory storage devices. As explainedearlier, the confinement of an optical material in a resonantmicrocavity affects the decay rate. Depending on whether the cavity isin resonance with the transition energy of the optical material or not,the lifetime is either decreased or increased. It is therefore possibleto significantly enhance the lifetime of the material and to use thiseffect to store information.

[0150] Another possible way to store information with a resonantmicrocavity would be based on hole burning. This process and itsapplication for the storage of information is well known. By putting thematerial in a resonant microcavity one could not only use the enhancedabsorption but also the earlier described effect of increased lifetimeto make the hole burning process more efficient.

[0151] RMDs could also be used in the design of light valves. This wouldrequire two RMD's. One RMD without a phosphor would be grown on top of aRMD with a phosphor. The first RMD would modulate the intensity of thelight emanating from the second RMD. The modulator would work by tuningthe first RMD to its resonant frequency or tune it away from itsresonant frequency. The process of tuning the first RMD (using theelectro-optic or the piezo-electric effect) would be achieved byapplying a voltage to the first RMD. This modulator could also be usedas a switch by turning the light completely on and completely off. Amodulated RMD 421 grown on substrate 422 is shown in FIG. 22. In thisfigure, RMD 421, comprising mirrors 424, 428 with active layer 426between, is modulated by applying a voltage V to mirror layers 424 and428. This modulation can be accomplished using either electro-optic orpiezo-electric effects.

[0152] The ability to tune the cavity resonance using, for example,electro-optic or piezo-electric effects, would allow the RMD to beutilized in a variety of communication modes. Resonant microcavitiescould be designed to emit light and receive light over a range offrequencies and solid angles. These frequencies and solid angles couldbe modified by applying electric signals. Thus RMDs could be used tosend and receive information. Friend or foe identifiers used in militaryequipment would be one possible use.

[0153] Using RMD's in a Plasma Display Panel could also be used to builda fluorescence lamp. Compared to common fluorescence lamps the RMD lamphas the advantage of strongly enhanced fluorescence which results in agreater efficiency. A single RMD lamp would emit light of a certainwavelength. This is useful for applications such as stage-lamps. Commonstage lamps emit over the UV, the visible and the infrared region anduse filters to select a certain wavelength (color). This filter-processmakes the lamp very inefficient since most of the light is not allowedto exit the lamp. In contrast, the RMD lamp creates only light of acertain wavelength and does therefore not require a filter. Theefficiency is therefore much higher. The combination of a R, G and Bdevice would result in a white light source.

[0154] In general, any light source can in principle be substituted bythe resonant microcavity display. For example, incandescent lights aretypically filtered to produce colored lights for car tail lights andtraffic signal lights. Resonant microcavities can replace these currentlight sources with highly efficient single color and directional lightsources. Excitation could use any of the means already discussed.

[0155] In non-emissive displays, the light source and image producingsurface are separate. The image is typically formed using a light valvewhich modulates the light produced by the light source. A common lightvalve display uses a combination of one or more liquid crystals andpolarizers and forms what is called a liquid crystal display (LCD).Light valves are used in both reflection and transmission and find usein both projection and direct view applications. The pixel size isdetermined solely by the light modulator.

[0156] In each application, a sufficiently bright light source isrequired. Often the display also requires full color capability.Currently for flat panel application, a fluorescence lamp is used as abacklight and creates the white light that is then modulated by a LCDpanel. To create a full color flat panel display, color filters areinserted at each pixel to filter the white light and generate the threeprimary colors.

[0157] The RMD can be incorporated in such flat panel displayapplications and form the light source. For a monochromatic display, themodulator would be attached to one large area resonant microcavity. Themicrocavity can be excited by any one of the three excitation means.Full color would be best generated by an array of microcavitiesconsisting of alternating stripes in which each striped region isconstructed to form one continuous resonant microcavity designed togenerate one color.

[0158] For projection devices, an arc lamp is used to generate a whitelight source and the color is typically generated by using dichroicfilters to separate the three primary color components of the whitelight. Instead, the three colors can be produced by three independentresonant microcavities or by producing an array of microcavities.

[0159] In addition, a LCD modulator requires the input light to beinitially polarized and uses a polarizer located in the input. It ispossible to eliminate this polarizer by designing the resonantmicrocavity to generate polarized light. This can be accomplished in anumber of ways. For example, the region between the mirrors can befabricated using birefringent material in such a manner the cavity willresonate at different frequencies depending on the polarization of thelight. The cavity can be designed so that only one polarized lightcomponent will resonate at the desired frequency.

[0160] The principal advantage of using resonant microcavities togenerate the light used for light modulators is the increased lightoutput efficiency. The RMD light source will produce high brightnesslevels and is highly directional. The latter is particularly useful forLCD applications since the input light must be contained within acertain range of solid angle. In addition, the elimination of colorfilters and dichroic beam splitters will increase the overall throughputThe other engineering advantage is the compact nature of the RMD whichis particularly useful for flat panel applications.

[0161] In one illustrative embodiment shown in FIG. 23, a monochromaticflat panel display 270 is depicted. In this example, the resonantmicrocavity 275 is excited by UV light generated by a plasma discharge282 excited by a source 284 of AC power. Any damaging UV that leaks outpast microcavity 275 is absorbed by substrate 274; another UV blockingsubstrate 286 may also be used on the other side of plasma discharge282. The light valve uses an LCD 272 to modulate the light. LCD 272 canbe addressed in a number of modes and this specification is not affectedby using the resonant microcavity. The key design considerations for themicrocavity would involve the divergence of the light, the lightpolarization, brightness and resonance wavelength.

[0162] The above embodiments are given as illustrative examples and arenot intended to impose any limitations on the invention.

What is claimed is:
 1. A luminescent display, comprising a resonantmicrocavity with an active region, the active region having a phosphordisposed therein for emitting light.
 2. The luminescent display of claim1 , wherein said microcavity comprises means for modifying a processselected from the group consisting of spontaneous emission processes ofthe phosphor and energy transfer processes of the phosphor.
 3. Theluminescent display of claim 1 , wherein said microcavity comprisesmeans for modifying the spontaneous emission processes of the phosphor.4. The luminescent display of claim 1 , wherein said microcavitycomprises means for modifying energy transfer processes of the phosphor.5. The microcavity of claim 2 , wherein the resonant microcavity istunable.
 6. The luminescent display of claim 5 , and further comprisingelectro-optic means for tuning the resonant frequency of saidmicrocavity.
 7. The luminescent display of claim 5 , and furthercomprising piezo-electric means for tuning the resonant frequency ofsaid microcavity.
 8. The luminescent display of claim 2 , wherein saidmicrocavity comprises means for producing, in said phosphor, anelectromagnetic field having a substantially modified electric fieldamplitude when said phosphor is excited by an energy source relative tothe amplitude of an electric field produced in said phosphor were saidphosphor disposed in free space and excited by the same energy source.9. The luminescent display of claim 8 , wherein said electromagneticfield having said substantially modified electric field amplitude is astanding wave electromagnetic field.
 10. The luminescent display ofclaim 9 , wherein the standing wave electromagnetic field has anantinode inside said microcavity, and said phosphor is disposed in aregion including said antinode.
 11. The luminescent display of claim 9 ,wherein the standing wave electromagnetic field has a plurality ofantinodes inside said microcavity, and said phosphor is disposed in aregion including said plurality of antinodes.
 12. The luminescentdisplay of claim 9 , wherein the standing wave electromagnetic field hasa node inside said microcavity, and said phosphor is disposed in aregion including said node.
 13. The luminescent display of claim 9 ,wherein the standing wave electromagnetic field has a plurality of nodesinside said microcavity, and said phosphor is disposed in a regionincluding said plurality of nodes.
 14. The luminescent display of claim9 , wherein the phosphor comprises a dopant within the microcavitydisposed in a region of the microcavity having a substantially modifiedelectric field amplitude.
 15. The luminescent display of claim 8 ,wherein said microcavity is dimensioned to produce a travelingelectromagnetic wave having the substantially modified electric fieldamplitude.
 16. The luminescent display of claim 2 , wherein themicrocavity comprises a structure selected from the group consisting ofcoplanar microcavities, three dimensional microcavities, andcombinations thereof.
 17. The luminescent display of claim 16 , whereinthe microcavity comprises a structure selected from the group consistingof confocal microcavities, hemispherical microcavities, and ringcavities.
 18. The luminescent display of claim 8 , wherein saidmicrocavity is excitable to establish the substantially modifiedelectric field amplitude inside said microcavity.
 19. The luminescentdisplay of claim 18 , wherein said display is excitable by electrons.20. The luminescent display of claim 19 further comprising a cathode raytube to generate exciting electrons for exciting said active layer. 21.The luminescent display of claim 19 and further comprising a filamentcathode means disposed behind said microcavity in a vacuum for emittingelectrons to excite said active layer.
 22. The luminescent display ofclaim 21 and further comprising a control grid disposed between saidfilament cathod means and said microcavity.
 23. The luminescent displayof claim 19 and further comprising a high-voltage field emission devicefor exciting said active layer.
 24. The luminescent display of claim 19and further comprising field-emissive material disposed within saidmicrocavity to cause emitted electrons to tunnel from said emissivematerial into said phosphor, thereby exciting said phosphor; and aconductive layer disposed within said microcavity for conducting emittedelectrons to ground.
 25. The luminescent display of claim 24 in whichsaid active region is disposed in a region of the substantially modifiedelectric field amplitude within said microcavity.
 26. The luminescentdisplay of claim 18 wherein said active region is excitable by anelectric field and further comprising means for exciting said activeregion with an electric field.
 27. The luminescent display of claim 18wherein said active region is excitable by electromagnetic radiation andfurther comprising means for exciting said active region withelectromagnetic radiation.
 28. The luminescent display of claim 27wherein the means for exciting said active region comprises a laser. 29.The luminescent display of claim 27 wherein the means for exciting saidactive region comprises means for generating a plasma discharge.
 30. Theluminescent display of claim 2 in which the resonant microcavitycomprises thin films.
 31. The luminescent display of claim 2 wherein themicrocavity comprises: (a) a substrate; and (b) a structure disposedupon said substrate comprising an active region and a plurality ofreflective regions.
 32. The luminescent display of claim 31 , whereinthe luminescent display comprises a plurality of said microcavities,each having a resonant region therein, and said microcavities areoperatively coupled to form a larger resonant region.
 33. Theluminescent display of claim 31 wherein the plurality of reflectiveregions comprise: (a) a front reflective region disposed upon saidsubstrate, and (b) a back reflective region; and the active region isdisposed between the front and the back reflective regions.
 34. Theluminescent display of claim 33 in which the front reflective region,the active region, and the back reflective region comprise thin films.35. The luminescent display of claim 31 wherein the substrate, theactive region, and plurality of reflective regions are each comprised ofinorganic materials.
 36. The luminescent display of claim 31 wherein thesubstrate comprises an organic material.
 37. The luminescent display ofclaim 31 wherein the active region comprises an organic material. 38.The luminescent display of claim 31 wherein the plurality of reflectiveregions comprise an organic material.
 39. The luminescent display ofclaim 31 wherein the substrate, the active- region, and the plurality ofreflective regions each comprise organic materials.
 40. The luminescentdisplay of claim 31 , in which said substrate and said structuredisposed thereon are flexible.
 41. The luminescent display of claim 31wherein at least one of said plurality of reflective regions comprises awavelength-dependent reflector that is substantially reflective within anarrow wavelength bandwidth and substantially transmissive outside ofsaid narrow wavelength bandwidth.
 42. The luminescent display of claim41 and further comprising an opaque surface behind one of saidwavelength-dependent reflectors to increase display contrast.
 43. Theluminescent display of claim 31 wherein said reflective regions comprisedielectric reflectors.
 44. The luminescent display of claim 43 whereinsaid dielectric reflectors further comprise a plurality of alternatingparallel layers, wherein layers comprising a material with a relativelylow index of refraction alternate with layers comprising a material witha relatively high index of refraction.
 45. The luminescent display ofclaim 44 wherein said material with a relative low index of refractioncomprises a material selected from the group consisting of fluorides andoxides.
 46. The luminescent display of claim 44 wherein said materialwith a relatively high index of refraction comprises a material selectedfrom the group consisting of sulfides, selenides, nitrides, and oxides.47. The luminescent display of claim 31 wherein at least one of saidplurality of reflective regions comprises a metallic reflector.
 48. Theluminescent display of claim 31 wherein said active region comprises aphosphor selected from the group consisting of sulfides, oxides,silicates, oxysulfides, and aluminates.
 49. The luminescent display ofclaim 48 wherein said phosphor includes an activator comprising amaterial selected from the group consisting of transition metals, rareearths, substances having color centers, and combinations thereof. 50.The luminescent display of claim 31 wherein the thickness of the activeregion is equal to a selected wavelength of light to be emitted by thedisplay multipled by an integer and divided by the quantity 4 times theindex of refraction for light of the selected wavelength in a materialcomprising the active region.
 51. The luminescent display of claim 31wherein the microcavity comprises a plurality of active regions and thethickness of the plurality of active regions is equal to a selectedwavelength of light to be emitted by the display multipled by an integerand divided by the quantity 4 times the index of refraction for light ofthe selected wavelength in a material comprising the plurality of activeregions.
 52. The luminescent display of claim 31 wherein the thicknessof the active region is equal to a selected wavelength of light to beemitted by the display multipled by an integer and divided by thequantity 2 times the index of refraction for light of the selectedwavelength in a material comprising the active region.
 53. Theluminescent display of claim 31 wherein the microcavity comprises aplurality of active regions and the thickness of the plurality of activeregions is equal to a selected wavelength of light to be emitted by thedisplay multipled by an integer and divided by the quantity 2 times theindex of refraction for light of the selected wavelength in a materialcomprising the plurality of active regions.
 54. The luminescent displayof claim 2 wherein said resonant microcavity comprises a photonic bandgap material.
 55. The luminescent display of claim 31 , in which atleast one of the pluralty of reflective regions comprises a photonicband gap crystal.
 56. The luminescent display of claim 31 , in which theactive region comprises a photonic band gap crystal.
 57. The luminescentdisplay of claim 31 and further comprising means for generating apredetermined angular light distribution from light emitted from saidactive region.
 58. The luminescent display of claim 57 in which saidmeans for generating the predetermined angular light distributioncomprises a structure selected from the group consisting of lenses,diffusers, holographic elements, gradient index elements, andcombinations thereof.
 59. The luminescent display of claim 57 whereinsaid means for generating a predetermined angular light distribution isdisposed within said substrate.
 60. A method of generating a controlledcolor, directional light beam comprising the step of exciting theluminescent display of claim 1 to emit light from said microcavity. 61.A modulated light source, comprising: (a) the luminescent display ofclaim 1 ; (b) means for exciting said phosphor in said active region;and (c) light valve means in front of said microcavity for modulatinglight emitted from said microcavity.
 62. The modulated light source ofclaim 61 , wherein said resonant microcavity is adapted to emitpolarized light.
 63. A method of producing a luminescent displaycomprising the step of growing a resonant microcavity containing anactive region inside said microcavity, in which a phosphor is grown inthe active region.
 64. The method of claim 63 wherein the step ofgrowing a resonant microcavity comprises a growing process selected fromthe group consisting of physical vapor deposition and chemical vapordeposition processes.
 65. The method of claim 63 and further comprisingthe additional step of selecting the phosphor, active layer thickness,cavity quality factor, and cavity type to control phosphor decay time.66. The method of claim 63 and further comprising the additional step ofselecting the phosphor, active layer thickness, cavity quality factor,and cavity type to control chromaticity of light emitted from thedisplay.
 67. The method of claim 63 wherein the growth of the activeregion is controlled so that the thickness of the active region is equalto an integer number of quarter wavelengths of light corresponding to aselected chromaticity.
 68. The method of claim 63 wherein the width ofthe active region is controlled so that the thickness of the activeregion is equal to an integer number of half wavelengths of lightcorresponding to a selected chromaticity.
 69. The method of claim 63comprising an additional step of selecting the phosphor, active layerthickness, cavity quality factor, and cavity type so that a resonancefrequency of the microcavity lies within a natural chromaticity of thephosphor in the active region.
 70. The method of claim 63 wherein themicrocavity is grown on a substrate selected to maximize the heattransfer efficiency of the display.
 71. The method of claim 63 andfurther comprising the additional step of selecting the phosphor, activelayer thickness, cavity quality factor, and cavity type to control thedirectionality of light emitted from the display.
 72. A method ofproducing a luminescent display comprising the step of using holographicphotolithography to produce a resonant microcavity containing an activeregion inside said microcavity.
 73. A communications device comprising:(a) a tunable resonant microcavity; and (b) an active region disposedwithin said microcavity and comprising a phosphor.
 74. Thecommunications device of claim 73 , and further comprising electro-opticmeans for tuning the resonant frequency of said microcavity.
 75. Thecommunications device of claim 73 , and further comprisingpiezo-electric means for tuning the resonant frequency of saidmicrocavity.
 76. An information recording device comprising: (a) aresonant microcavity; and (b) an active region disposed within saidmicrocavity and comprising a photosensitive material.